Fixed argument name for Command handlers

[HalfWhitt]

Changed the first non-self parameter name in Command handlers to command.

The ActionHandler protocol is defined as accepting self, command: Command, **kwargs, where command is the command that triggered the action. In the codebase, handlers were defined with this parameter named app and in one case widget. Especially since none of the defined handlers reference the argument, this is an entirely semantic fix with no runtime significance.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Good catch - thanks!